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Wednesday, August 31, 2011

Hurricanes

On his second voyage to the New World, Columbus encountered a tropical storm. Although his vessels suffered no damage, this experience proved valuable during his fourth voyage when his ships were threatened by a fully developed hurricane. Columbus read the signs of an approaching storm from the appearance of a southeasterly swell, the direction of the high cirrus clouds, and the hazy appearance of the atmosphere. He directed his vessels to shelter. The commander of another group, who did not heed the signs, lost most of his ships and more than 500 men perished.

Origins. A hurricane starts its life as a tropical disturbance, an area from 100 to 200 miles in diameter marked by atmospheric convection, resulting in cloud formation, and a discrete character distinct from a normal weather front. It becomes a tropical depression when it takes on a rotary character and wind speeds reach up to 33 knots. A tropical storm is clearly spinning and wind speeds reach up to 63 knots. With speeds above that, a tropical storm becomes a hurricane.

The word hurricane is derived from the Carib god Hurican, called Hurakan by the Mayans, whose breath blew the primordial sea from the land, creating dry Earth. Because of its Carib/Mayan origins, hurricane is the name given to storms in the North Atlantic and the Eastern North Pacific. In the Western North Pacific, they're called typhoons, except in the Philippines, where they're sometimes called baguios. In the Indian Ocean hurricanes and tropical storms are called cyclones or cyclonic storms. Such a storm originating in the Timor Sea is called a willy-willy.

Formation. No one is sure exactly what causes hurricane to form. There are several factors that are present in most hurricane formation:

water temperatures of at least 79.7 degrees F

rapid cooling with height that provides the energy for the hurricane

high humidity

low contrary winds (that would stall the storm's formation)

a distance at least 5 degrees of latitude from the equator (to allow the Earth's Coriolis effect to create spin)

a pre-existing tropical disturbance

By no means does a hurricane form every time these factors are present, and sometimes they form when some of the factors are not present.

Hurricanes and Climate Change. Some evidence suggests that the intensity of Atlantic hurricanes has increased in the last fifteen years, possibly due to increase water vapor in the atmosphere and higher sea surface temperatures. Despite this, the number of hurricanes worldwide has not increased. Major studies in Nature and Science predict stronger hurricanes over the next century.

Mariners and Hurricanes. Entire books have been written for mariners on hurricanes and hurricane avoidance. Most of the time, the smartest course of action is to avoid the storm entirely, a choice made more possible by the advent of increasingly accurate predictions using computer models and satellite monitoring. There are entire companies devoted to weather routing for ships. This is not and has not always been possible, however, and mariners who encounter a hurricane never forget it. In "The Typhoon Lady" (published in the US Naval Institute Proceedings in June 1949) Lieutenant (j.g) Robert J. Lauer described the encounter of the light aircraft carrier USS San Jacinto with a typhoon in September 1944

The intensity of the storm could never been imagined beforehand. Winds of over 100 knots and seas 70 to 80 feet high lashed at the ships of the formation. To ease the ferocious pounding the course was adjusted to place the ships in the trough of the seas, and speed was reduced to the minimum required to maintain steerageway. All hands stood by their respective spaces to ensure their security.
It was a terrifying sight to watch the gigantic breakers on the crest of the seas looming up, sometimes as much as thirty degrees above the horizontal, as the ship rolled through forty degrees or more. It would have been suicide to venture onto the flight deck.

Despite its reputation, even the so-called "eye of the hurricane" is not safe. Visibility increases and winds decrease, but seas can be "monstrous" and come from any direction. As "Bowditch" says of these storms, "The awesome fury of this condition can only be experienced."